Merkel cell carcinoma (MCC) is an aggressive, polyomavirus-associated skin cancer with an increasing incidence and a 5-year disease-associated mortality of 46%. Despite this growing health impact, very few clinical trials have focused on this unique disease and no targeted therapies exist. The Merkel cell polyomavirus (MCPyV), discovered in 2008, is involved in the pathogenesis of approximately 80% of MCC tumors. MCC typically requires persistent expression of immunogenic polyomavirus tumor-antigen (T-Ag) oncoproteins for growth and survival, making them ideal targets for immune therapy. Several lines of evidence suggest the importance of immune function in MCC. Patients with profound T-cell dysfunction have 10- to 30-fold increased MCC risk and their MCC sometimes spontaneously regresses following improvement in immune function. We have previously demonstrated that intratumoral CD8+ T cell infiltration is associated with excellent survival regardless of extent of disease at presentation. Although T cell immunosuppression is associated with increased MCC risk, >90% of MCC patients have no known immune suppression, suggesting a role for local immunosuppression in the tumor microenvironment. Indeed, we have recently found that most MCC tumors have prominent down-regulation of class-I HLA, a known mechanism for allowing tumor cells to escape host immunosurveillance by evading CD8+ T lymphocytes. We have shown that HLA-I down-regulation can be reversed in MCC by intratumoral interferon injection or by a single dose of radiation. Ongoing expression of viral oncoproteins combined with reversibility of immune evasion mechanisms in MCC present exciting opportunities to develop rational therapy for this often-lethal cancer. Collaborations between Drs. Nghiem, Koelle, Yee, and Chapuis have resulted in treatment of the first MCC patient with MCPyV-specific polyclonal T cells in conjunction with HLA-I up-regulating adjuvants. Responses include significant tumor shrinkage, persistence of MCPyV-specific T cells at several-fold above baseline, and improved antigen-specific T cell responses over 100 days after T cell infusion. In Aim 1, a phase I/II trial will examine the safety and efficacy of therapy with polyclonal antiviral T cells of a desired specificity/phenotype infused following HLA-I up-regulating pre-treatment. Availability of a large panel of MCPyV peptide-HLA tetramers will allow polyclonal antiviral T cells to be expanded and then tracked in vivo. Although highly promising and currently feasible, adoptive T cell therapy is costly and patient-specific. In Aim 2, we will develop tools and explore the potential for future off-the-shelf recombinant T cell receptor-based therapies by creating a panel of MCPyV-specific soluble T cell receptors. These versatile tools will be used to determine the baseline and induced expression of viral peptide/HLA complexes on MCC cells and could be readily developed into tumor-targeted delivery vehicles for stimulatory cytokines, radioactivity emitters, or emerging therapeutics.